Some neurons in the CNS at cortical, diencephalic and subdiencephalic levels have been shown to exhibit changes in spontaneous discharge with corresponding changes in the animal's state of arousal. We propose to identify and study, in the awake, behaving monkey, thalamic somatosensory neurons showing similar changes in spontaneous activity, as well as, those showing changes in responsiveness to somatic stimuli related to the level of arousal. It is our hypothesis that at least two distinct classes of neurons will be found in the ventral posterior portion of the thalamus. One group, the state independent somatosensory (AIS) neurons, will have discrete, usually contralateral receptive fields and show little or no variation in stimulus responsiveness or receptive field size as the monkey's state of arousal ranges from sleep to drowsy to awake and alert. The other class, called state dependent somatosensory (ADS) neurons, will show changes in unit excitability as a function of the animal's level of arousal. These changes may include increases in spontaneous discharge with increased arousal, or altered responsiveness to a somatic stimulus, or both. The responses of single neurons to somatic stimuli will be recorded in the ventrolateral and posterior thalamus of African green monkeys during three behaviorally and electroencephalographically defined states of arousal. Several measures of neuronal responsiveness will be made by an on-line computer and significant changes in unit activity that are associated with changes in arousal will be determined. Focal electrical stimulation of lemniscal and/or spinothalamic input at the midbrain level will be employed to determine the extent to which arousal related changes in thalamic somatosensory excitability are mediated indirectly by modulation at subthalamic levels or by acting directly at the thalamic level. In addition, functional lesions of the somatosensory thalamus and electrical stimulation of the thalamic reticular nucleus will be used to determine whether these structures are responsible for the production of arousal-related modulation by direct action at the thalamic level.